(a) Field of the Invention
The present invention relates to an ink jet recording head such as for recording characters or pictures in an ink jet recording device and, more particularly, to an improvement of the ink jet recording head for iteratively ejecting larger-volume ink droplets with a higher stability. The present invention also relates to a method for designing such an ink jet recording head.
(b) Description of the Related Art
Ink jet recording devices using a drop-on-demand scheme attract higher attentions in these, days. Such ink jet recording devices; are disclosed in Patent Publication 53-12138 and described in Laid-Open Publication JP-A-10-193587, for example. In the described devices, a pressure wave generator such as piezoelectric actuator generates a pressure wave in a pressure chamber, ejecting ink droplets through the nozzles communicated to the pressure chamber. FIG. 1 exemplarily shows in ink jet recording head in a conventional ink jet recording device. A pressure chamber 61 is communicated with a nozzle 62 for ejecting ink droplets 67, and an inlet port 64 for receiving therethrough ink from an ink reservoir (not shown) via a common ink passage 63. A diaphragm 65 is provided on the bottom of the pressure chamber 61.
For ejecting ink droplets, a piezoelectric actuator 66 provided on the bottom of the pressure chamber 61 generates a displacement for the diaphragm 65, which in turn generates a volume change for the pressure chamber 61, generating a pressure wave in the pressure chamber 61. The pressure wave allows part of the ink received in the pressure chamber 61 to be ejected outside the pressure chamber 61 through the nozzle 62 as an ink droplet 67. The ejected ink droplet 67 falls onto a recording medium such as a recording sheet, thereby forming an ink dot on the recording sheet.
Iterative formation of the ink dots based on supplied data generates images such as characters and pictures on the recording sheet. The piezoelectric actuator 66 is applied with a driving voltage among driving voltages having a variety of waveforms depending on the volume of the ink droplets to be ejected. A large-volume ink droplet generally used for recording characters or dark images is ejected by applying the driving voltage having a waveform such as shown in FIG. 2.
The driving waveform has a rising edge 151 for raising the voltage applied to the piezoelectric actuator 66 thereby reducing the volume of the pressure chamber 61 for ejection of the ink, a flat level having a voltage V1 and a falling edge 152 for recovering the original voltage level or the normal voltage Vb.
FIGS. 3A to 3F are sectional views of a nozzle in the ink jet recording head, consecutively showing the ink meniscus in the vicinity of the nozzle during application of the driving waveform. The meniscus 72 has a flat surface prior to volume reduction of the pressure chamber, as shown in FIG. 3A, then moves toward outside the nozzle 71 due to the volume reduction of the pressure chamber, ejecting an ink droplet 73, as shown in FIG. 3B. After the ejection of the ink droplet 73, the volume of the ink inside the nozzle 71 is reduced, forming a concave meniscus surface shown in FIG. 3C. The meniscus 72 then recovers the original shape due to the surface tension of the ink, as shown consecutively in FIGS. 3D to 3F.
FIG. 4 shows the positional profile of the meniscus surface shown in FIGS. 3C to 3F at the center of the meniscus with the elapsed time xe2x80x9ctxe2x80x9d just after the ink ejection. As shown in the drawing, the meniscus surface largely retracts toward the position y=xe2x88x9260 xcexcm at the time instant t=0, and eventually recovers to the original position y=0, or the position at the outer edge of the nozzle, after some vibration due to the function of the surface tension of the ink.
The recovery movement of the ink meniscus after the ejection of ink droplet is referred to as xe2x80x9crefillxe2x80x9d or xe2x80x9crefill operationxe2x80x9d in this text. The time length tr for the meniscus to recover the original position y=0, or the outer edge of the nozzle, after the ink ejection is referred to as a xe2x80x9crefill timexe2x80x9d in this text.
In iterative ejection of the ink droplets by using the ink jet recording head, an ejection operation should be effected after the completion of the refill operation resulting from the prior ejection in order to obtain a constant volume or a constant velocity of the ink droplet. That is, if the next ejection is effected before completion of the refill of the prior ejection, a stable iterative ejection cannot be obtained.
The factors largely affecting the maximum ejection frequency of the ink jet recording head include the refill time tr as described above and the number of nozzles. A larger number of nozzles increase the number of dots to be formed in a unit time length, thereby improving the maximum ejection frequency. In view of this fact, a conventional ink jet recording device is of a multi-nozzle type wherein a plurality of ejectors are juxtaposed and coupled together.
FIG. 5 shows a conventional multi-nozzle ink jet recording head. An ink reservoir 97 is communicated with a common ink passage 93, which is in turn communicated with a plurality of pressure chambers 91 via respective inlet ports (not illustrated). This arrangement allows the plurality of ejectors to eject ink droplets at a times thereby reducing the time length seeded for printing.
It is to be noted that the common ink passage 93 should be suitably designed in order to obtain a stable iterative ejection in the ink jet recording head. More specifically, for example, cross-talk of the pressure should be prevented between the ejectors which are communicated with the common ink passage. In addition, the difference in the ejection characteristics between the ejectors should be also reduced, the ejection characteristics depending on the positions of the connection to the common ink passage. In this respect, it is important that the common ink passage have a sufficient acoustic capacitance. Some head structures satisfying the above conditions have been proposed heretofore.
For example, JP-A-56-75863 describes an ink jet recording head including a common ink passage having a volume defined based on the volume of the pressure chambers. Each of JP-A-5249034 and JP-A-10-24568 describes the structure of a common ink passage accompanied with air damper for obtaining a larger acoustic capacitance for the common ink passage having a small size. JP-A-59-26269 describes a quantitative definition of the acoustic capacitance (or impedance) needed for the common ink passage. As described in these publications, a sufficient acoustic capacitance of the common ink passage prevents mutual interference between ejectors, thereby achieving a stable and uniform ink ejection among the plurality of ejectors communicated with the common ink passage.
Even if the above-described conditions are satisfied in the conventional multi-nozzle ink jet recording heads, however, a stable ink ejection is not always achieved depending on other factors, as detailed below.
The first case of the unstable ink ejection arises when a plurality of ejectors eject relatively large ink droplets at the same time with a higher frequency. In this ejection, the volume of the ejected ink droplet is unstable: large-volume ink droplets and small-volume ink droplets are alternately ejected, for example. In addition, the velocity of the ejected ink droplet is also unstable. An excessively unstable droplet velocity may cause that the nozzle receives air bubbles in the ink and eventually results in a non-ejection problem.
FIG. 6 shows the stability of the ink ejection obtained by changing the volume of the ink droplet and the ejection frequency in a conventional ink jet recording head. The stability is evaluated based on the change of the droplet velocity. As shown, when 32 ejectors in number simultaneously ejected ink droplets having a volume of 25 pico-litters (or 25xc3x9710xe2x88x9215 m3), the droplet velocity was unstable at frequencies above 11 kHz, and exhibited non-ejection at frequencies above 18 kHz. Observation of the ejection by a stroboscope revealed frequent occurrences of a case wherein large-volume droplets and small-volume droplets were ejected alternately at ejection frequencies above 11 kHz. In another case, the droplet volume and the droplet velocity were changed at random. When a larger droplet volume of 30 pico-litters was selected, similar results were observed at frequencies above 9 kHz.
The unstable ink ejection as described above was scarcely observed in the ejection of a small-volume droplet or ejection of a larger-volume droplet at a lower frequency. This means that a sufficient suppression of cross-talk was achieved, which in turn means that a sufficient acoustic capacitance was obtained for the common ink passage. The unstable ink ejection was also scarcely observed when the number of ejectors operating at the same was small. It was confirmed that all the ejectors communicated with the common ink passage revealed similar instability. These results of observations lead to a conclusion that the instability of the ink ejection did not result from the cross-talk. This necessitated investigation of the new factors of the unstable ejection, which were not considered heretofore, as well as the solution of the unstable ejection.
The problem of the unstable ejection may be a bar against developments of ink jet recording heads because the ink jet recording heads are requested to have a higher printing speed, which is attempted by an increase of the number of ejectors and of the ejection frequency, as well as an increase of the volumes of the ink droplets, i.e. expansion of the modulation range of the volume of the ink droplets.
The second case of the unstable ink ejection arises when an ink having a higher viscosity is used. FIG. 7 shows the stability of the ink ejection obtained by using inks having different value for the viscosity. When an ink having a viscosity of 3 mPaxc2x7s was used, the stability of the ink ejection was lost at frequencies above 11 kHz in the case of 32 ejectors simultaneously ejecting ink droplets having a volume of 25 pico-litters. When an ink having a viscosity of 6 mPaxc2x7s was used, the stability of the ink ejection was lost at frequencies above 6 kHz in a similar case. Observation of the ink ejection by a stroboscope revealed frequent occurrences of a case wherein large-volume droplets and small-volume droplets were ejected alternately, similarly to the case of attempting ejection of a large-volume droplet. Thus, it is considered that the instability of the ink ejection caused by using an ink having a higher viscosity results from a reason similar to the reason which raises the instability of the iterative ejection of ink droplets having a large volume. It is confirmed that a higher ink viscosity increases the instability of the ink ejection.
Current developments of the ink jet recording head highlight the increase in the ink viscosity because the demand for a high-performance ink is increasing. The development of the high-performance ink is directed to improvement in the recording performance of the current ink with respect to the regular sheet as well as a ultra-high printing speed thereon. This may be achieved partly by the increase of the ink viscosity.
The higher ink viscosity, however, prevents the ink jet recording head from ejecting ink droplets having a large volume at a higher frequency, as described before, thereby raising a problem in practical introduction of the ink having a higher viscosity. Thus, the suppression of the unstable ink ejection is one of the most important subjects for the ink jet recording head, in the view point of practical introduction of such a high-viscosity ink as well.
It is an object of the present invention to provide an ink jet recording head for use in an ink jet recording device which is capable of suppressing unstable ink ejection when a plurality of ejectors simultaneously eject large-volume ink droplets at a higher frequency and thus being adapted to a high-speed printing.
It is another object of the present invention to provide an ink jet recording head for use in a recording device which is capable of suppressing unstable ink ejection during ejecting a high-viscosity ink and being adapted to a variety of ink viscosities.
It is a further object of the present invention to provide a method for designing such an ink jet recording head.
The present invention provides an ink jet recording head including an ink supply system having an ink reservoir and a common ink passage communicated to the ink reservoir, a plurality of pressure chambers each-communicated to the common ink passage, each of the pressure chambers including an ink nozzle for ejecting ink from a corresponding one of the pressure chambers, wherein a flow resistance r [Ns/m5] of the ink supply system generated at a static ink flow satisfies the following relationship:
xe2x80x83r less than 800/(qxc2x7Nxc2x7f),
wherein q, N and f represent a droplet volume [m3] of an ink droplet ejected by each of the nozzles at a time, a number of the pressure chambers and an ejection frequency for ejecting the ink droplets, respectively.
The present invention also provides a method for designing an ink jet recording head having an ink supply system including an ink reservoir and a common ink passage communicated to the ink reservoir, a plurality of pressure chambers each communicated to the common ink passage, each of the pressure chambers including an ink nozzle for ejecting ink from a corresponding one of the pressure chambers, the method including the step of determining a flow resistance of the ink supply system during a static flow in the ink supply system to suppress a refill time for each of the nozzles down to below a specified ejection frequency designed for the nozzles.
In accordance with the inkjet recording head of the present invention and the ink jet recording head designed by the method of the present invention, a stable iterative and simultaneous ejection by a plurality of ejectors can be obtained at a higher ejection frequency as well as fort the case of an ink having a higher viscosity.